Composition comprising diethyl phosphite and a copolymer of acrylonitrile



Patented May 6, 1952 COMPOSITION COMPRISING DIETHYL PHOSPHITE AND A COPOLYMER OF ACRYLONITRILE George E. Ham, Dayton, Ohio, assignor, by mesne assignments, to The Chemstrand Corporation, a corporation of Delaware No Drawing. Application September 23, 1949,

Serial No. 117,505

3 Claims.

This invention relates to a new method of preparing synthetic fibers from copolymers of acrylonitrile. More particularly the invention relates to a new solvent for acrylonitrile polymers, and to completely miscible mixtures including acrylonitrile copolymers, from which mixtures quality fibers can be extruded.

It is well known that copolymers of over 75 percent acrylonitrile are capable of being fabricated into high strength fibers. The conventional technique for preparing fibers from these copolymers involves the dissolution of the copolymer in a suitable solvent and thereafter extruding the viscous solution so prepared through an orifice into a medium which removes the solvent and precipitates the acrylonitrile copolymer in a continuous form. Many solvents have been proposed but many of them are impracticable due to the cost and due to the tendency of the solution to gel upon standing, especially at elevated temperatures encountered in the spinning operation.

The" primary purpose of this invention is to provide a new low cost solvent for the preparation of synthetic fibers. A further purpose of this invention is to provide a solvent which forms more stable mixtures or solutions of acrylonitrile copolymers. A still further purpose of the invention is to provide a method of forming fibers of high tensile strength and desirable elongation.

It has been found that diethyl phosphite is an excellent solvent for copolymers of 70 to 90 percent by weight of acrylonitrile and from ten to 30 percent of other polymerizable monomers, for example vinyl acetate and other vinyl esters of monocarboxylic acids, methyl methacrylate and other alkyl esters of methacrylic acid, ethyl acrylate and other alkyl esters of acrylic acid, methacrylonitrile, vinylidene chloride, ethyl maleate and other alkyl esters of maleic acid, ethyl fumarate and other alkyl esters of fumaric acid, styrene and other vinyl substituted aromatic compounds, a-methyl styrene and other isopropenyl aromatic hydrocarbons, vinyl chloride and other vinyl halides, vinyl pyridine and other vinyl substituted heterocyclic amines, and other polymerizable monomers capable of copolymerization with acrylonitrile. Diethyl phosphite is also a solvent for copolymers of 75 to 90 percent by weight of acrylonitrile and a plurality of other monomers, for example the copolymers of acrylonitrile, methacrylonitrile and vinyl acetate as described and claimed in copending application In the practice of this invention, as it is in the preparation of all acrylonitrile fibers, the molecular weight of the polymer is of critical importance. The polymer should have a molecular weight in excess of 10,000, and preferably in excess of 25,000. These molecular weights are determined by measuring the viscosity of the polymer when dissolved in a suitable solvent, such as dimethylformamide, in the manner well known to the art. It is also very desirable to use acrylonitrile copolymers which are substantially uniform throughout with respect to the chemical composition and physical structure. Such uniform copolymers enable the practice of the invention more economically, permitting the utilization of continuous uninterrupted spinning and greatly minimizing fiber fractures and clogging of the spinnerets.

In the practice of this invention the polymers of acrylonitrile are used in finely divided form. Although massive polymers may be ground to desired particles, preferably solvent-non-solvent polymerization procedures are employed in the preparation of the polymer. states of the polymer obtained by spray drying the emulsions, or by filtration and subsequent drying of the solid polymers enable them to be used directly. The finely divided polymer is mixed with diethyl phosphite in any type of mixing device, such as a dough mixer or a homogenizer. It is desirable to use a solution of as high a concentration of the polymer as possible, but the maximum concentration is dependent upon the molecular weight of the polymer.

To obtain fibers of optimum physical properties polymers of molecular weights in excess of 25,000 are used, and when using such polymers it is only possible to dissolve from five to 35 percent in the diethyl phosphite without exceeding practicable viscosity values. Although as low as five percent of the polymer can be used in spinning operations, such concentrations are undesirable because they necessitate the removal and recovery of too much solvent from the extruded solution, thereby increasing solvent recovery cost and reducing spinning speeds by reason of the longer periods required for coagulation. The concentration of the polymer in the solution is preferably between seven and 25 percent but will ultimately be determined by considering the desired physical properties of the fiber and the speed of spinning, which speed is a The subdivided function of the concentration and viscosity of the polymer solution. The viscosity will depend upon the chemical composition and the molecular weight of the polymers. The optimum proportions can best be determined by selecting a uniform molecular weight polymer having good fiber forming properties and dissolving it in the smallest amount of the diethyl phosphite necessary to form a viscous solution capable of extrusion at convenient temperatures The fibers are spun by extruding the diethyl phosphite solution of the acrylonitrile polymer through an orifice, or a spinneret having a plurality of orifices, into a medium which removes the solvent. The volume of the solution passed through the spinneret per unit of time must be constant in order to produce a fiber of uniform size. This is best achieved by using a positively driven gear pump constructed of corrosion resistant metals, such as stainless steel, and adapted to deliver a constant flow ofsolution re- .gardless of minor changes in viscosity and regardless of the resistance oifered by the spinneret. It is also desirable to pass the solution through one or more filters before reaching the spinneret in order to remove all possible traces of foreign matter and particles of incompletely dissolved polymer. The polymer solution may be delivered to the gear pump .by means of pressure applied by an inert gas to the liquid surface of the solution reservoir, which must be heated to maintain the solution fluid enough to pass through the conduits. The gear pump, filter devices and conduits to the spinneret are preferably heat insulated and may be heated to maintain the body of solution in liquid state, The extruding operation should be conducted at elevated temperatures, but far enough below the boiling point of the solvent to prevent bubbles or other irregularities in the fiber.

The medium into which the solution is extruded and which removes the solvent may be either liquid or gaseous. The method involving the use of liquids is known as wet spinning and usually utilizes aqueous media, which may have salts or other water soluble compounds dissolved there in. The spin bath ma be any liquid which is a non-solvent for the acrylonitrile polymer, but which either dissolves the diethyl phosphite, or converts it into soluble compounds. The solvent is leached out of the stream of polymer solu--' tion, which first becomes a viscous stream and finally a solid filament. When a spinneret with a plurality of apertures is used the several streams of polymer converge and ultimately form a single fiber. The spin bath must necessarily bepf sufficient size to permit the complete, or substantially complete, removal of the diethyl phosphite. Obviously the rapidity of extrusion will affect the size of the spin bath,'high speeds requiring muchlonger baths. The temperature of the bath also affects the size, higher temperatures permitting more rapid difiusion oi"- 'the diethyl phosphite from the fiber 'andenabling the use of shorter baths l The wet spinning method of fiber preparation is particularly adaptable to the use of diethyl phosphite as the polymer solvent. Because of the slower rate of solution of diethyl phosphite in water as compared to the rate of solution of other known solvents, for example N,N-dimethylformamide l. I, l\ -dimethylacetamide, it is possible to obtain clear, transparent fibers, whereas, the fibers prepared from solutions in conventional solvents, on precipitation in water, are

other defects.

1 move the solvent. This method operates at higher temperature; and the diethyl phosphite is evaporated from the surface of the fiber. The maximum temperature to which the fibers can .be subjected is approximately the boiling point of the diethyl phosphite, since evaporation within the body of the fiber may cause bubbles or The fiber may be heated by convection from the hot gaseous medium or by radiation from the Walls of the spinning cell. Generally a combination of both convection and radiation is involved, but methods involving principally radiation are generally more efiicient and permit the operation with the wall temperature considerably in excess of the boiling point of the diethyl phosphite. The evaporation of the diethyl phosphite from the fiber surface and the speed of the fiber prevent the development of a temperature exceeding that to which the fiber is stable to decomposition. The dry spinning method is particularly useful at high rates of extrusion.

In general the methods of both Wet and dry spinning commercially used are adaptable for spinning from diethyl phosphite solutions, but special considerations may be involved due to the different chemical nature of diethyl phosphite. Automatic machinery for spinning continuously, drying the thread if necessary, and winding it on suitable spools maybe modified with the teaching of this specification. As in the case of most synthetic fibers, the fibers of acrylonitrile copolymers spun from diethyl phosphite solutions may be stretched to develop optimum physical properties. If desired part of the necessary stretching may be accomplished in the spinning medium by drawing the fiber out of the bath at a rate more rapid than the rate .of extrusion.

Further details of the invention are set forth with respect to the following examples.

Example 1 A copolymer of percent by weight of acrylonitrile and 15 percent vinylacetate was prepared by polymerization in an aqueous medium. The polymer was dispersed in nine parts of diethyl phosphite and heated to C. A transparent solution was thereby obtained.

Example 2 Using a copolymer of 95 percent by weight of acrylonitrile and five percent of vinyl acetate, which was prepared by polymerization in an aqueous medium, an attempt to prepare diethyl phosphite solution was made. A ten percent dispersion of the polymer in diethyl phosphite was heated to C. for a prolonged period but no transparent solution was obtained.

The invention is defined by the following 2,595,847 6 homogeneous miscible mixture of 65 to 95 percent by weight of diethyl phosphite and from five to REFERENCES CITED percent of a copolymer of from 7 to 90 percent The foliowing references are of record in the by weight of acrylonitrile and from ten to 30 file Of @1118 patent; percent of vinyl acetate. 5

3. A new composition of matter, comprising a UNITED STATES PATENTS homogeneous miscible mixture of to 95 per- Number Name Date cent by weight of diethyl phosphite and from five 2,224,695 Prutton Dec. 10, 1940 to 35 percent of a copolymer of from to 2,503,245 COOVer et a1 p 1, 1950 percent by weight of acrylonitrile and from ten 10 to 30 percent of vinyl pyridine.

' GEORGE E. HAM. 

1. A NEW COMPOSITION OF MATTER, COMPRISING A HOMOGENEOUS MISCIBLE MIXTURE OF 65 TO 96 PERCENT BY WEIGHT OF DIETHYL PHOSPHITE AND FROM FIVE TO 35 PERCENT OF A COPOLYMER OF FROM 70 TO 90 PERCENT BY WEIGHT OF ACRYLONITRILE AND FROM TEN TO 30 PERCENT OF ANOTHER COPOLYMERIZABLE MONOMER. 